1. Field of the Invention
This invention relates to a timing control means used in connection with an excitation circuit having multiple branches, which are driven by a common switch, where the branches of the excitation circuit introduce variable timing delays between the voltage output by each of the branches without relative timing jitter.
2. Description of Related Prior Art
Several applications involving pulsed gas laser systems require controlling the timing between two separate discharges used for the excitation of a laser, e.g., preionization and main discharge or spiker and sustainer discharge. This control can also be necessary in systems having two separate lasers, e.g. oscillator and power amplifier or injection locking of large pulsed lasers to low power tuneable and single mode lasers. Commonly these tasks have been accomplished using two separate excitation circuits, driven by individual high voltage switches. The relative timing between these circuits can then be continuously adjusted and delayed by synchronized trigger generators.
While modern digital electronics can provide accurately timed trigger sources with continuously variable delays on the nanosecond and subnanosecond time scale and with sub-nanosecond jitter, high voltage switches employed in pulsing systems for laser discharge excitation are inherently subject to switching time jitter.
Switching time jitter relates to the time interval between the application of a trigger pulse to a high voltage switch, and the actual switching of the high voltage switch. This time interval is called the delay time. This time interval is not absolutely stable. Within the high voltage switch, such as a thyratron or spark gap, a highly conducting plasma must develop. The plasma formation includes avalanche type electron multiplication. This plasma formation depends on the starting conditions and causes variations in the delay time. The statistically varying portion of the delay time is called switching time jitter. For example, the spark gap high voltage switches and thyratons are subject to switching time jitter on the order of 10-100 ns for the former and 1-6 ns for the latter.
In order for a thyratron to be switched with an accuracy approaching one nanosecond, expensive, specifically selected thyratrons or multiple grid tubes have to be employed and trigger circuits have to drive the grids with fast rising, kilovolt trigger pulses. High voltage switching, using solid state switches, yields switching time jitter similar to thyratrons. An additional source for time jitter is the voltage dependent switching delay of excitation circuits, particularly for circuits employing magnetic pulse compression stages. Charging voltage regulation of better than 0.1% is generally required if a time jitter in the nanosecond range is required.
The problem of relative switching time jitter can be eliminated by employing a common high voltage switch for both discharge circuits. While this will eliminate the timing jitter, it will also remove the ability of continuously tuning the relative timing, as can be obtained with separately switched circuits, since only fixed time delays can be designed into the excitation circuits.
Magnetic switches, utilizing saturable inductors, have been employed in laser excitation circuits to provide magnetic pulse compression of the excitation pulses. The time delay jitter introduced by variations of the charging voltage in saturable inductor stages can be controlled on the nanosecond time scale, by either precise regulation of the charging voltage or by accurately measuring the charging voltage on a shot to shot basis and then adjusting the trigger time of the switching element by an external delay generator in order to compensate for voltage fluctuations. For a discussion of this approach to controlling time delay jitter, see, Patent No. DE 3842492 A1, filed Dec. 12, 1988, inventor Dr. Dirk Basting; and D. L. Birx, E. G. Cook, S. Hawkins, A. Meyers, L. L. Reginato, J. A. Schmidt and M. W. Smith, "Regulation and drive system for high rep-rate magnetic pulse compressors" Proceedings 15th Power Modulator Symposium, Baltimore, Md., Jun. 14-16, 1982, pp 15-21.
Variable nanosecond time delay in thyratron and spark gap trigger circuits have been realized in the past by using variable bias inductors in low power switching trigger circuits of high voltage switches. (See, D. L. Birx et al., "Regulation and drive system for high rep-rate magnetic pulse compressors", 15th Power Modulator Symposium, Baltimore, Md., 1982; and H. J. Baker and N. Seddon, "Magnetic switching circuits for variable high voltage pulse delays and gas laser synchronization", Journal of Physics E: Scientific Instruments, Volume 19, 1986, pp 149-152) These references provide for using variable bias inductors to control the trigger timing of separately switched elements and are still subject to the jitter of the high voltage switching elements. Today this technique could be practiced using modern digital circuits.